World-wide demand for petroleum products has increased dramatically in recent years, depleting much of the known, high value, light crude oil reservoirs. Consequently, production companies have turned their interest towards using low value, heavy oil in order to meet the ever increasing demands of the future. However, because current refining methods using heavy oil are less efficient than those using light crude oils, refineries producing petroleum products from heavier crude oils must refine larger volumes of heavier crude oil in order to get the same volume of final product. Unfortunately though, this does not account for the expected increase in future demand. Further exacerbating the problem, many countries have implemented or plan to implement more strict regulations on the specifications of the petroleum-based transportation fuel. Consequently, the petroleum industry is seeking to find new methods for treating heavy oil prior to refining in an effort to meet the ever-increasing demand for petroleum feedstocks and to improve the quality of available oil used in refinery processes.
In general, heavy oil provides lower amounts of the more valuable light and middle distillates. Additionally, heavy oil generally contains increased amounts of impurities, such as sulfur, nitrogen and metals, all of which generally require increased amounts of hydrogen and energy for hydroprocessing in order to meet strict regulations on impurity content in the final product.
Heavy oil, which is generally defined as bottom fraction from atmospheric and vacuum distillatory, also contains a high asphaltene content, high sulfur content, high nitrogen content, and high metal content. These properties make it difficult to refine heavy oil by conventional refining processes to produce end petroleum products with specifications that meet strict government regulations.
Low-value, heavy oil can be transformed into high-value, light oil by cracking the heavy fraction using various methods known in the art. Conventionally, cracking and cleaning have been conducted using a catalyst at elevated temperatures in the presence of hydrogen. However, this type of hydroprocessing has a definite limitation in processing heavy and sour oil.
Additionally, distillation and/or hydroprocessing of heavy crude feedstock produce large amounts of asphaltene and heavy hydrocarbons, which must be further cracked and hydrotreated to be utilized. Conventional hydrocracking and hydrotreating processes for asphaltenic and heavy fractions also require high capital investments and substantial processing.
Many petroleum refineries perform conventional hydroprocessing after distilling oil into various fractions, with each fraction being hydroprocessed separately. Therefore, refineries must utilize the complex unit operations for each fraction. Further, significant amounts of hydrogen and expensive catalysts are utilized in conventional hydrocracking and hydrotreating processes. The processes are carried out under severe reaction conditions to increase the yield from the heavy oil towards more valuable middle distillates and to remove impurities such as sulfur, nitrogen, and metals.
Currently, large amounts of hydrogen are used to adjust the properties of fractions produced from conventional refining processes in order to meet the required low molecular weight specifications for the end products; to remove impurities such as sulfur, nitrogen, and metal; and to increase the hydrogen-to-carbon ratio of the matrix. Hydrocracking and hydrotreating of asphaltenic and heavy fractions are examples of processes requiring large amounts of hydrogen, both of which result in the catalyst having a reduced life cycle.
Petroleum continues to be the dominant source for supplying the world's energy needs. However, with increased concern on air quality, world governments have urged producers to remove impurities, in particular, sulfur compounds, from petroleum streams. In particular, transportation fuels (gasoline and diesel) are required to be almost free from sulfur compounds (approximately less than 10 wt ppm sulfur). In order to meet such strict regulation on sulfur contents of transportation fuels, ultra deep desulfurization is generally carried out with distilled stream or cracked stream, which have boiling point ranges for gasoline and diesel.
Generally, desulfurization of the petroleum fraction (distilled & cracked stream) can be achieved by catalytic hydrotreatment in the presence of high pressure hydrogen gas. For heavier fractions of petroleum, catalytic hydrocracking and catalytic hydrotreatment is typically applied with very high pressures of hydrogen in order to convert high molecular weight hydrocarbons to low molecular weight ones, thereby meeting boiling point range requirements for transportation fuels. Catalysts for hydrotreatment and hydrocracking suffer from deactivation caused mainly by poisonous matters contained in feedstock and coking. Hence, high pressures of hydrogen are used to maintain the catalyst life. However, catalysts have certain life time in hydrotreatment and hydrocracking. Therefore, catalysts have to be replaced regularly and frequently. Additionally, the large quantities of hydrogen consumed during hydrotreatment and hydrocracking represent a significant disadvantage, as hydrogen is one of the most important and valuable chemicals in the refining and petrochemical industry.
Non-catalytic and non-hydrogenative thermal cracking of petroleum stream is also used for removing impurities. However, these types of refining processes are only capable of modest impurity removal. Moreover, these processes generally result in a significant amount of coke.
Another option to produce clean transportation fuels is using sweet crude oil having fewer amounts of impurities, in particular, sulfur compounds. By using sweet crude oil, complicated and intensive hydrotreatment and hydrocracking can be carried out with lower operating costs. However, the supply of sweet crude oil is fairly limited, while sour crude oil is found in much larger quantities.
As an alternative to conventional catalytic hydrotreatment/hydrocracking and thermal cracking, contacting hydrocarbons in the presence of supercritical water is beginning to garner more attention. In the prior arts, supercritical or near critical water has been employed as a reaction medium to remove impurities and also crack large molecules into small ones without generating a large amount of coke. However, reactions occurring in supercritical water medium are not clearly identified yet.
The critical point of water is 374° C. and 22.06 MPa. Properties of water change dramatically near critical point. The dielectric constant of water changes from around ∈=78 at ambient condition to around ∈=7 at critical point. Furthermore, small changes of temperature and pressure in supercritical conditions result in wide variation of dielectric constant of water (∈=2-30). Such a wide range of dielectric constants covers non-polar organic solvent such as hexane (∈=1.8) and polar organic solvent such as methanol (∈=32.6). The density of water also changes dramatically at near critical points. At supercritical condition, density of water varies from 0.05 to 0.3 g/ml. Furthermore, supercritical water has much lower viscosity and high diffusivity than subcritical water.
Unique properties of supercritical water have been utilized for facilitating certain reactions. For example, high solubility of organic matters and oxygen gas in supercritical water is utilized for decomposing toxic waste materials (Supercritical Water Oxidation=SCWO).
Hydrocarbon molecules contained in petroleum stream are also more easily dissolved in supercritical water although solubility of hydrocarbon depends on its molecular weight and chemical structure. High temperature condition of supercritical water (>374° C.) generates radical species from hydrocarbon molecules, which are more easily converted to various hydrocarbons through complicated reaction networks. In general, termination through bi-radical reactions cause dimerization followed by coke generation. On the other hand, a hydrocarbon molecule carrying radicals are easily decomposed to smaller ones. Generally speaking, inter-molecular radical reaction generates larger molecules such as coke while intra-molecular radical reaction generates smaller molecules. The generation of a large quantity of coke in conventional thermal cracking of petroleum stream is caused by such inter-molecular radical reaction, whereas the presence of supercritical water as a reaction medium reduces inter-molecular radical reaction by cage effect, thereby facilitating intra-molecular radical reactions such as decomposition and isomerization. Therefore, the use of supercritical water allows for the petroleum stream to be converted to a lighter stream with negligible amount of coke.
Impurity removal is also possible with aid of supercritical water; however, the prior arts teach that supercritical water is more effective in decreasing viscosity than in desulfurization.
For example, Atsushi Kishita et al. (Journal of the Japanese Petroleum Institute, vol. 46, pp. 215-221, 2003) treated Canadian bitumen with supercritical water by using batch reactor. After 15 minute reaction at 430° C., the viscosity of bitumen decreased drastically from 2.8×104 mPa*S to 28 mPa*S, while the sulfur content decreased only from 4.8 wt % sulfur to 3.5 wt % sulfur. The amount of coke generated by the disclosed treatment was 9.6 wt % of feed bitumen.
Limited performance of supercritical water in removing impurities, in particular, sulfur, from petroleum stream is attributed to the limited availability of hydrogen. Although higher operating temperatures are certainly beneficial to improve desulfurization performance, heavy-duty reactor material and large quantities of energy are required to reach such high operating temperatures, e.g., over 450° C.
Feeding hydrogen with the petroleum stream is also beneficial to improve desulfurization. Hydrogen can be supplied by hydrogen gas or other chemicals which can generate hydrogen through certain reaction. For example, carbon monoxide can generate hydrogen by water gas shift reaction. Also, oxygen can be used to generate hydrogen through oxidation of hydrocarbons included in petroleum stream and following water gas shift reaction. However, injecting high pressure gases along with the petroleum stream and water causes many difficulties in handling and safety. Additionally, chemicals such as formaldehyde, can also be used to generate hydrogen through decomposition; however, adding chemicals in with the supercritical water decrease process economy and leads to greater complexities.
Therefore, it would be desirable to have an improved process for upgrading oil with supercritical water fluid that requires neither an external supply of hydrogen nor the presence of an externally supplied catalyst. It would be advantageous to create a process and apparatus that allows for the upgrade of the oil, rather than the individual fractions, to reach the desired qualities such that the refining process and various supporting facilities can be simplified.
Additionally, it would be beneficial to have an improved process that did not require complex equipment or facilities associated with other processes that require hydrogen supply or coke removal systems so that the process may be implemented at the production site.